DESCRIPTION: (Adapted from the application) Body fluid homeostasis depends on reflexes which act to modulate the rate of renal water and sodium loss and on ingestive behaviors (i.e., thirst and salt appetite) that corrects deficits. Although renal mechanisms can slow fluid loss, the restoration of vascular volume depends on the ingestion of water and solute (e.g., sodium). The maintenance of extracellular volume requires that the CNS receives and processes information about the status of body water and sodium. Several visceral sensory systems are known to provide this afferent input but there is only a very limited understanding about how this information is handled by the CNS. The present proposal builds upon the investigator s prior studies on the central processing of afferent signals involved in body fluid and cardiovascular homeostasis. The proposed research will employ a model of rapid-onset sodium appetite in the rat. Because of its short latency of induction, this model of experimental sodium intake is especially appropriate for use in conjunction with a range of neurophysiological/pharmacological methods that permit the investigation of brain pathways and processes subserving extracellular fluid volume regulation. Experiments using the rapid-onset sodium appetite model will focus on defining the role of serotonergic mechanisms in the lateral parabrachial nucleus (LPBN) that we have implicated in the regulation of extracellular fluid volume. The proposed studies employing functional (behavioral), pharmacological, electrophysiological and neuroanatomical methods are designed to lead to converging experimental findings to increase our understanding of how the brain processes information necessary for maintaining body fluid and cardiovascular homeostasis. Such new information has relevance for the well-being of normal individuals exposed to physiological (exercise) and environmental (heat) challenges and for understanding mechanisms underlying pathological conditions related to fluid balance (e.g., hypertension, congestive heart failure, and renal disease).